1. Technical Field
This invention relates to the evaporative casting process (ECP), and to the art of measuring permeability and compactability of sand molds.
2. Description of the Prior Art
In ECP, a form or pattern of the item to be cast is made, as shown in FIG. 1. An evaporative pattern is advantageous because it produces an exact duplicate of the desired casting, preferably in expanded bead polystyrene. Complex patterns are usually made in sections to facilitate defining internal passages and contouring; such sections are then glued together to produce a completed pattern. Several of the patterns may be glued to a common sprue of a gating system to produce a cluster. Such cluster of patterns is coated with a refractory wash which acts as a thermal barrier between the molten metal and the unbonded sand mold to prevent sand burn-in and heat loss of the molten metal. The wash usually contains a finely ground refractory material, such as aluminum, zirconium, or silica flour, which is emulsified and suspended in a carrying agent such as water or alcohol.
The washcoated cluster is then placed in an oven to dry; after drying, it is set into a flask which is filled with free-flowing sand. The flask is essentially a pouring box which will contain loose molding material; such pouring box is either open-ended at its top and/or perforated at its sides to permit the migration of gases during and after the pouring of molten metal. The molding sand is unbonderized and free-flowing to permit the compaction of such sand around the delicate foam pattern after it is suspended in the flask and not before. The unbonded sand can be agitated either by aeration, using air or other gas, or the flask itself may be vibrated, both methods being effective to reduce the angle of repose of the sand as close as possible to zero, thus allowing the sand to flow into and fill all areas, including the inner and outer cavities of the pattern. Angle of repose means the angle of a cone formed by pouring the dry, unbonded sand medium onto a flat surface. The lower the angle, the closer the material to a liquid, which essentially takes the shape of the container into which it is poured. It is important that the sand be compacted or densified to provide support for the weight of the liquid metal to be poured into the flask and which displaces the foam pattern.
Molten metal is poured into the flask directly into the foam sprue or leader with the result that molten metal will evaporate the pattern progressively and displace it, as shown in FIG. 2. The formation of gases due to the evaporation of the foam pattern allows the unbonded molding material to remain in position. After the liquid metal has solidified, the casting and sand are dumped out of the flask and the casting is then sent to a cleaning room to be cleaned and readied for shipment.
The sand must have high permeability and good packing characteristics. This seemingly antagonistic set of objectives for the sand must be attained. Sand must not only be compacted to a degree sufficient to withstand the forces of the hot molten metal poured thereinto, assisted by the pattern gases, but must also be able to satisfactorily permit the migration of the newly generated gases from the evaporated pattern through the mold and out through the flask. Thus, the sand mold must have the proper amount of channels therein to facilitate this migration of gases.
Vibration of the flask or sand is an essential part of ECP. If vibration is not sufficient, the internal passages and voids of the foam pattern will not be filled properly and as a result the sand may cave in before the molten metal has a chance to fill the void left by the evaporated foam pattern. On the other hand, if vibration is excessive, the compactness of the sand will reduce the permeability of the mold and block the gases generated by the foam evaporation escaping through the sand to atmosphere. Too much vibration may also distort the pattern or even break the delicate foam pattern at certain locations.
How to control and measure the sand compaction level during vibration, particularly in real time, is the purpose of this invention.
Sand compactability of a mold body has been heretofore determined usually by destructive techniques (see U.S. Pat. Nos. 3,608,357; 4,555,934; 4,671,100; and 3,638,478).
Compactability in the industry has been traditionally measured by taking a sample of sand either before or after preparation of the molding. Generally, a prepared sample is taken somewhere between the mixer and the molding machine. The sample is screened or fluffed into a standard cylinder and raked level on the top. The sand is then rammed three times with a two kilogram weight. The percent compactability is computed by measuring the travel of the ram. Such measurement is actually taking a ratio of noncompacted volume to the compacted volume. However, for a given amount of material, the percentage change in volume will usually be equal to the percentage change in density. Compactability, which usually is between 30-55% for most foundry operations, is a measure of how much the sand can be compacted during the molding process. Compactability may be expressed as a ratio of the difference between the compacted sand density and the noncompacted sand density to the compacted said density.
The prior art has also used numerous other techniques for measuring other physical characteristics of the sand, which characteristics will hopefully tell how to predict compactability of the sand. Such characteristics have included sand chemistry, its grain size or shape, and moisture. These techniques are disclosed in U.S. Pat. Nos. 2,679,317; 2,890,347; 3,136,010; 3,460,030; 3,534,260; 3,693,079; and 3,600,574. However, even if all such other physical characteristics were measured for the sand being transferred to the mold, such information cannot and will not tell the operator the actual compaction of a specific mold, much less its entire, total compactability. To do so would require determining all the random distributions and locations of the grains in their locked network, not by an arbitrary selection of one location in the entire mold.